counting reps
 

When working out should you stop at a certain number of reps per set or should you keep going until you tire the muscle that is being worked?

that would depend on what your goal is .

Powerlifting there goal on the platform is 1 rep. so in the gym they don't usually go over 5 reps per set for the mains

bodybuilding is different again. higher reps for most. but how high ? guess that depends on the weight youre using and the number you pick.

either way I think its good to have a set number of reps as it helps judge gains. ie: keep the reps the same while increasing the weight.